Currently, the developmental trend of the processes for producing grain-oriented silicon steel is directed to heating of slab at relatively low temperature. A process for producing grain-oriented silicon steel at medium temperature using aluminum nitride and copper as inhibitors may realize the relatively low temperature for heating slab (1250-1300° C.). This process adopts double cold rollings with complete decarburizing annealing therebetween, wherein the complete decarburizing annealing (to reduce carbon to below 30 ppm) is carried out after the first cold rolling, and the resultant steel is rolled to the thickness of steel sheet with the second cold rolling before it is coated with MgO annealing separator as it is or after it is recovery annealed at low temperature, followed by high-temperature annealing and post treatment. In order to form complete glass film at the stage of high-temperature annealing, the conditions for decarburizing annealing in the process of heating slab at medium temperature have to be controlled strictly to form an appropriate oxide layer on the surface. However, the slab to be decarburizing annealed between the two cold rollings is rather thick. Under the decarburizing annealing conditions which can ensure formation of an appropriate oxide layer, carbon can not be reduced to below 30 ppm. Furthermore, the oxide layer on the surface is damaged during the second cold rolling after decarburizing annealing, throwing an impact on the surface quality.
In the art of grain-oriented silicon steel production, an underlying layer is known as a layer (such as 2MgO.SiO2) formed by reaction between an annealing separator layer (such as MgO) with oxide layer formed during decarburization. The underlying layer is also referred as “forsterite layer” or “glass film layer” in the art. In the production of grain-oriented silicon steel, it has always been difficult to form a good underlying layer that guarantees the tension effect and the insulating effect of tension coating. However, the unevenness at the joint of the underlying layer and the substrate may hinder magnetic domain activity, leading to an increase of iron loss. On the other hand, the existence of the glass film underlying layer results in poor stamping performance of the grain-oriented silicon steel. In order to further lower iron loss and improve stamping performance, grain-oriented silicon steel without underlying layer has been developed recently.
According to the method disclosed in Chinese Patent 03802019.X, the composition of the slab based on mass comprises Si 0.8˜4.8%, C 0.003˜0.1%, acid soluble Al 0.012-0.05%, N 0.01% or less than 0.01%, balanced by Fe and unavoidable inclusions. After hot rolling, the resultant hot rolled sheet is formed to the final thickness of the sheet via single cold rolling or two or more times of cold rollings with middle annealing therebetween as it is or after it is annealed. Subsequently, in an atmosphere with an oxidability that will not render formation of oxides of Fe family, the steel sheet is subjected to decarburizing annealing. After an oxide layer comprising silicon oxide as the main component is formed on the steel sheet surface, an annealing separator comprising aluminum oxide as the main component is coated to make a mirror-like surface of the annealed steel sheet. Secondary recrystallization is stabilized by controlling the moisture entrapped by the annealing separator which comprises aluminum oxide as the main component and is coated in the form of aqueous slurry and then dried, and by controlling the partial pressure of vapor during annealing the steel sheet.
According to the method disclosed in Korean Patent KR 526122, decarburization and nitridation are carried out concurrently in a process for producing silicon steel at low temperature, wherein magnesium oxide separator added with SiO2 and Cl is used to avoid formation of an underlying layer during high-temperature annealing. This method is characterized by the following features. The composition of the billet based on weight comprises C 0.045-0.062%, Si 2.9-3.4%, P 0.015-0.035%, Als (acid soluble Al) 0.022-0.032%, Cu 0.012-0.021% N 0.006-0.009%, S 0.004-0.010%. The temperature at which the billet is heated is controlled in the range of 1150-1190° C. After cold rolled to the thickness of steel sheet, the steel sheet is decarburized and annealed at 840-890° C. in a protective atmosphere of wet nitrogen and hydrogen containing ammonia. A separator comprising 100 parts by weight of MgO+3-12 parts by weight of SiO2+25 parts by weight of chloride ions as the main components is used for high-temperature annealing.
The above two patents are directed to grain-oriented silicon steel without underlying layer. They both use (Al, Si) N or AlN+MnS as inhibitors, and adopt a conventional high-temperature or low-temperature production process in which the billet is cold rolled to the thickness of steel sheet before decarburizing annealing, for the purpose of further lowering iron loss and improving stamping performance.
A continuous secondary recrystallization annealing process without inhibitors is disclosed in Chinese Patent CN 1400319, wherein the composition of molten steel based on weight comprises C 0.08% or less, Si 1.0-8.0%, Mn 0.005-3.0%; and the steel sheet is subjected to hot rolling, cold rolling, recrystallization annealing, secondary recrystallization annealing, decarburizing annealing and continuous high-temperature annealing sequentially. Grain-oriented electromagnetic steel sheet with high magnetic flux density and low iron loss is produced by this process without using inhibitors.